1. Field of the Invention
The present invention pertains to valves and particularly to spool valves which are opened and closed by longitudinal sliding of a valve element in the bore of a valve body. Even more specifically, the invention pertains to such a valve which will close automatically upon a drop in the pressure of the fluid which it is communicating so that it may be used as a check valve. For example, where fluid under pressure is being passed from a first conduit or vessel to a second conduit, the fluid flow should be cut off if there is a break in the second conduit. The valve of the present invention can be used to provide for automatic cut-off of the fluid flow in such situations.
2. Description of the Prior Art
In one type of prior art check valve, a piston-like valve element is mounted in a valve body. The valve element has a small diameter bleed passageway which permits limited fluid flow at low velocities but which provides sufficient resistance to high velocity fluid flow that the piston-like valve element is moved thereby from an "opened" to a "closed" position. Such a velocity increase would occur for example, if the downstream should drop due to a break in the outlet line or the like. In the "open" position, a large diameter passageway carries the bulk of the fluid flow. When the valve element is moved to the "closed" position, the large diameter passageway is sealed off. This type of valve is disadvantageous in that the small diameter bleed passageway precludes complete closure. Thus there is, of necessity, leakage of the fluid being handled.
In another type of prior art check valve, a ball-and-seat assembly closes off the fluid flow when the downstream pressure drops, and the fluid flow velocity rises. The valve may be re-opened by manually opening a by-pass to increase the downstream pressure. However, it is then necessary to manually reset the valve to its operating condition so that it may close. Thus the valve may be left locked open, due to negligence or the like, and dangerous and/or expensive leaks may occur.